Signalling pathways are composed of the ON mechanisms that generate internal signals and the OFF mechanism that remove these signals as cells recover from stimulation. Most attention will be focused on how second messengers and their downstream effectors are inactivated. The second messengers cyclic AMP and cyclic GMP are inactivated by phosphodiesterase (PDE). Inositol trisphosphate (InsP3) metabolism is carried out by both inositol trisphosphatase and inositol phosphatases. Diacylglycerol (DAG) metabolism occurs through two enzyme systems, DAG kinase and DAG lipase.
In the case of Ca2+ signalling, recovery is carried out by the Ca2+ pumps and exchangers that remove Ca2+ from the cytoplasm. The mitochondria also play an important role in Ca2+ homoeostasis.
Many of these second messengers activate downstream effectors through protein phosphorylation, and these activation events are reversed by corresponding protein phosphatases.
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